1. Field of the Invention
The invention relates in general to a method of forming interconnections, and more particularly to a method of forming bonding pads.
2. Description of the Related Art
As the requirements on the complexity and precision of an integrated circuit design keep increasing in order to reduce the feature size of a semiconductor device and increase the integration of an integrated circuit, a semiconductor device has to contain more than two metal layers to achieve the fabrication of high-density metal interconnects on a limited surface of a chip. As the design rules of a semiconductor device become finer and higher, the requirements of lower resistivity and higher reliability increase, as well.
Since copper has better conductivity and reliability than aluminum, copper has become a new material used in the metal layers. During a conventional fabrication of interconnections, a copper pad is exposed in the air by a bonding pad window. The oxidation rate of copper is very high so that copper is easily oxidized to form copper oxide. This increases the resistance of the bonding pad and decreases semiconductor device reliability. Furthermore, the bonding pad adheres badly to a sweat joint with a welding line in the conventional fabrication of interconnections. The yield of devices thus decreases due to the bad adhesion.
FIGS. 1A to 1E are schematic, cross-sectional views showing a conventional process of forming a bonding pad.
In FIG. 1A, a substrate 100 having a metal layer thereon is provided. An inter-metal dielectric layer 102 is formed on the substrate 100. A part of the inter-metal dielectric layer 102 is removed to form dual damascene opening 104 and a trench 106 therein.
In FIG. 1B, a conformal barrier layer 108 is formed on the inter-metal dielectric layer 102. A copper layer 110 is formed on the barrier layer 108 to fill the dual damascene opening 104 and the trench 106.
In FIG. 1C a chemical mechanical polishing (CMP) process is performed to remove a part of the copper layer 110 until the inter-metal dielectric layer 102 is exposed. A dual damascene structure 110a and a copper pad 110b are formed within the dual damascene opening 104 and the trench 106. A glue layer 112 and an aluminum layer 114 are formed on the inter-metal dielectric layer 102.
In FIG. 1D, a part of the aluminum layer 114 and a part of the glue layer 112 are removed using a photolithography and etching process to form an aluminum pad 114a on the copper pad 110b.
In FIG. 1E, a passivation layer 116 is formed on the structure shown in FIG. 1D. A part of the passivation layer 116 is removed to form a bonding pad window 118 and to expose the aluminum pad 114a.
The conventional method described above resolves the problem of bad adhesion between the copper pad and a welding line, but an additional mask is required to define the aluminum layer. The additional mask increases the manufacturing cost and complicates the fabrication process for forming bonding pads.